1. Field of the Invention
Exemplary embodiments of the present invention relate to a mixer for converting frequencies in a wireless communication system; and, more particularly, to a frequency mixer that performs a stable operation at a low voltage without being influenced by noise of power lines such as VDD and GND.
2. Description of Related Art
In general, a frequency mixer is one of essential elements in a communication system. Such a mixer converts a high frequency signal to a low frequency signal, or converts a low frequency signal to a high frequency signal.
In early days, a frequency mixer was equipped in a communication system for the purpose of safely transmitting voice, picture, or data in a long distance. Currently, the frequency mixer is essentially used for various purposes in wired or wireless communication systems. In addition to the original purpose of the frequency mixer, the frequency mixer has been used to improve a signal quality, to expand a transmission distance, to overcome an antenna size problem, and to manage channels to accommodate multiple users.
A typical frequency mixer generates a third frequency by mixing two frequencies. Such a typical frequency mixer is generally classified into a frequency mixer for transmitting a signal, and a frequency mixer for receiving a signal.
The frequency mixer for transmitting a signal generates a radio frequency (RF) signal by mixing an intermediate frequency (IF) and a local oscillator (LO) signal. The frequency mixer for receiving a signal generates an IF signal by mixing an RF signal with an LO signal.
Hereinafter, a circuit structure of a typical frequency mixer will be described with reference to FIGS. 1 and 2.
FIG. 1 is a circuit diagram illustrating a single balanced Gilbert mixer, and FIG. 2 is a circuit diagram illustrating a double balanced Gilbert mixer.
The single balanced Gilbert mixer of FIG. 1 uses a single RF signal. Unlike the single balanced Gilbert mixer of FIG. 1, the double balanced Gilbert mixer of FIG. 2 uses differential RF signals.
The double balanced Gilbert mixer of FIG. 2 has a structure more complicated than that of the single balanced Gilbert mixer of FIG. 1. However, the double balanced Gilbert mixer of FIG. 2 has superior isolation characteristics between ports such as RF to LO, LO to RF, RF to IF, and LO to IF. Furthermore, the double balanced Gilbert mixer of FIG. 2 can suppress the generation of spurious or harmonic through signal attenuation of 180° phase difference. Such characteristics of the double balanced Gilbert mixer can alleviate requirements for an external filter. Therefore, the double balanced Gilbert mixer of FIG. 2 has been used more often than the single balanced Gilbert mixer of FIG. 1.
Meanwhile, the isolation characteristic is one of major factors to determine the performance of a frequency mixer. If a signal of one of ports leaks to the other port, the leakage signal may be combined with itself, causing a self-mixing problem. The self-mixing problem generates DC offset that fluctuates an output DC value. The DC offset makes an entire system to abnormally operate. Therefore, the superior isolation characteristic between ports improves the performance of the entire system. Particularly, the isolation characteristic for isolating an RF signal or an LO signal is more important because the RF signal is easily coupled with signals at adjacent lines and an LO signal has the biggest signal level.
The typical frequency mixers shown in FIGS. 1 and 2 are very vulnerable to noise of power lines such as VDD and GND. Such noise is generated at and entered from a printed circuit board (PCB) due to conductive Electro Magnetic Interference (EMI) or radioactive EMI entered from external devices. For example, noises 110 and 210 entered through a VDD line is directly transferred to an output IF port through a current flowing through a load resistor RL. Noises 130 and 230 entered through a GND line is inputted to transistors MN1 and MN2 of an RF input port and the input noises 130 and 230 are also amplified and transferred to the output IF port. Accordingly, the frequency mixers of FIGS. 1 and 2 are very weak to noises of power lines.
Such a power noise problem can be reduced by adding a tail current source. Adding the tail current source can attenuate the influence of the noise entered through power lines. For example, in order to improve the isolation characteristic of an RF port in the frequency mixer of FIG. 2, a tail current source is disposed at the middle without directly connecting sources of RF input transistors MN1 and MN2 with a ground GND. Such a tail current source attenuates the influence of noise entered through the GND line. Therefore, the frequency mixer can obtain a perfect phase difference.
However, the tail current source may distort a signal. For example, a frequency mixer uses very low source voltage such as around 1 V. If a tail current source is added into such a frequency mixer using very low source voltage, the frequency mixer might lower a headroom of a signal. If a signal becomes comparatively little higher, the signal becomes easily saturated. Accordingly, the tail current source may be a burden to dispose in the frequency mixer in view of dynamic range or linearity (IP3) of the frequency mixer.
As described above, adding the tail current source to the frequency mixer is not a perfect solution to blocking the noises of power lines. Accordingly, the typical frequency mixers of FIGS. 1 and 2 are influenced directly from noises of power lines, which are entered from an external device. Such a power line noise is amplified by a variable gain amplifier (VGA) disposed behind the frequency mixer. The amplified noise increases noise figure (NF) and decreases tolerance of an entire system. Therefore, it is required to develop another solution to prevent the power line noise problem.
In addition, the frequency mixers of FIGS. 1 and 2 maintain an operating point by receiving a DC voltage such as RFBIAS and LOBIAS from an additional voltage bias circuit. However, the variability of such individual DC voltage bias is very high and the correlation among RF, LO, and DC bias has not been considered. Accordingly, a driving error of a frequency mixer becomes increased.
Furthermore, an additional voltage bias circuit generally includes resistors arranged in series at a power supply port. Or, the additional voltage bias circuit includes resistors and transistors or transistors only. That is, the voltage bias circuit has a structure easy to leak noise of a power line to a DC bias.
Therefore, there has been a need for developing a circuit to reduce a driving error between frequency mixers and to provide a bias voltage without being influenced by noise of power lines.